The present invention relates to the dewatering of sludges. More specifically, the present invention relates to the dewatering of difficult sludges, such as sludges derived from an autoheated thermophilic aerobic digestion system (ATAD).
Biosolids are a byproduct of waste water treatment which have been recognized as a valuable commodity. Before sludge can be returned into the natural cycle, an adequate treatment for stabilization and pathogen reduction is required. The United States Environmental Protection Agency has established standards for both goals. One highly effective technology for achieving a high degree of pathogen reduction is autothermal thermophilic aerobic digestion ("ATAD").
Such ATAD systems have been operating at full-scale in Europe for over twenty years. Currently, there are over fifteen systems in the United States and at least five in Canada. The significant advantages of this waste treatment system will lead to further installations throughout the world.
The ATAD process is an aerobic digestion process that operates in the thermophilic temperature range (40-80.degree.) without supplemental heating. The thermophilic bacteria flourish at these elevated temperatures and have a much higher metabolism rate. This results in a faster rate of soluble organic destruction, when compared to conventional aerobic digestion. As these bacteria destroy the volatile organics, they release energy in the form of heat. The soluble organics are reduced to their lowest components: CO.sub.2 and H.sub.2 O. Additionally, the high temperature of the system is effective in destroying pathogens. This process has the additional advantage of reducing the biomass volume. Up to 45% of the solids are destroyed after one week in an ATAD system. This degree of solids reduction is possible in an anaerobic digester after three weeks or in a conventional aerobic digester after two months.
Much interest has been generated over the past few years in the ATAD process and, as noted above, a number of ATAD systems are springing up in various parts of the United States. The reason is that the ATAD process is a cost effective method of achieving Class A standards of pathogen and vector attraction reduction as defined in the United States EPA 40 C.F.R. Part 503 sludge regulations. Class A sludge has reduced disposal restrictions compared with Class B sludge, which is the sludge produced from conventional aerobic and anaerobic digestion.
With more and more waste water authorities opting to treat sludge to Class A standards to ease disposal and simplify record keeping requirements, the ATAD process will undoubtedly become a more popular and widespread sludge stabilization technique in this country. In many cases, ATAD sludge is being dewatered to facilitate land application or other beneficial uses.
Unfortunately, although the ATAD process has served to be a beneficial digestion process, a major drawback of this type of digestion system is the inability to effectively dewater the sludge at a reasonable cost. On average, nearly a three-fold increase in polymer dosage is required to get effective flocculation and dewatering of ATAD sludges. For example, dewatering costs for conventional aerobically digested sludge ranges from $20-30/dry ton.
In contrast, a study conducted on behalf of an ATAD facility has reported that polymer dosage increased from $25/dry ton to an excess of $150/dry ton when the same polymer that was utilized previously for dewatering aerobically digested sludge was used for the dewatering of the ATAD sludge. See Burnett et al, "Dewaterability of ATAD Sludges," WEFTEC '97 Proceedings from Residual & Biosolids Management Vol. 2, p. 299-309 (1997). Indeed, the high polymer demand remained at this rate for several months, despite attempts at optimizing and reducing the polymer dose through conventional trial-and-error changes to feed point, holding time, and other parameters. Id. Because of the high cost of dewatering such ATAD sludges, many ATAD facilities are now forced to haul the sludge away to various land application sites for disposal.
Accordingly, a need exists for an improved method for dewatering difficult sludges, such as sludges derived from the ATAD process.